With the advancement of complex precision parts that can be made from aluminum alloys, it becomes increasing likely that the conventional fabrication processes will not meet the new requirement. As a result, various casting processes are being increasingly utilized to fabricate precision aluminum alloy parts.
In the so-called investment casting process, a precise wax pattern is prepared corresponding to the metal article to be formed. Then the wax pattern is invested (i.e., coated) with particulate ceramic material so as to build up a ceramic shell mold with a desired thickness. If the article to be manufactured contains complex openings or cavities, such as deep holes, bent holes, under-cuts, etc., one or more of the so-called ceramic cores can be used which will be disposed in places where such openings or cavities are to be formed. When the wax pattern is subsequently removed from the ceramic shell mold, the ceramic cores will remain in place so as to form appropriate openings, cavities or the like in the final article to be cast. Ceramic cores can also be used in other casting processes such as die casting, gravity casting, etc.
After the metal casting is completed, the ceramic cores must be removed from the cast article. Typically, the ceramic cores can be readily removed by forming the cores from a material which is soluble in caustic alkali. However, this procedure cannot be used for aluminum alloy objects which are subject to chemical attacks by the caustic alkali. Several ceramic core materials have been developed which are soluble or at least disruptable in water so that, after the aluminum alloy objects are formed, the ceramic cores can be removed by the use of water. However, commercially available water-soluble ceramic cores often do not exhibit acceptable mechanical properties that will allow them to withstand the severe autoclave conditions when making aluminum alloy castings.
U.S. Pat. No. 4,572,272, the content thereof is incorporated herein by reference, disclosed a ceramic core made from alumina and other non-silica based ceramic materials which is made leachable in fused anhydrous caustic alkali by the addition to the core of a material containing a hydrogen donor group such as hydroxyl groups, hydrides, or chemically combined water. The ceramic core disclosed in the '272 patent solved the deformation problems experienced from those silica-based cores; however, it is not water-soluble. And the need to use caustic alkali to remove the ceramic core after casting can cause waste disposal problems.
U.S. Pat. No. 4,925,492, the content thereof is incorporated herein by reference, disclosed a fired ceramic core which can be rendered water disruptible under the steam autoclaving conditions in aluminum investment casting. The ceramic core disclosed in the '492 patent contains 20 to 50 wt % of water-soluble salts and inert ceramic fillers, and are vacuum impregnated with ethyl silicate and a cured phenolic resin. The ceramic core disclosed in the '492 patent provides improved resistance during steam autoclaving to remove the wax mold; however, its high salt content often caused instability in the dimension of the final products.
U.S. Pat. No. 5,460,854, the content thereof is incorporated herein by reference, disclosed a method of strengthening a fired porous ceramic core for use in investment casting including the step of impregnating a fired porous ceramic core with an aqueous solution of a strengthening agent such as water-soluble gum, resin, and sugar. Preferably, the strengthening agent is polyvinyl alcohol. However, because the strengthening agent is applied after the porous ceramic core is formed, it provides strengthening function only during the wax-injecting step. After the ceramic shell is formed, its strength would be weakened and the ceramic shell became brittle.
Because of the increasing importance of aluminum alloy based precision parts and the shortcomings of the currently available ceramic cores, it is highly desirable to devote significant research efforts to develop improved ceramic cores that can be advantageously employed for the casting of precision aluminum alloy parts.